Switching inverter with sine wave output

ABSTRACT

A technique of producing a sine wave output from a solid state inverter using Triacs as switches to supply the sine wave output from a DC input.

United States Patent 21 Appl. No.1 76,910

[52] U.S. Cl. ..32l/9 R, 307/221 B, 302/223 B,

1 32l/DIG. l [5 l] Int. Cl. ..H02m l/l2 [58] Field ofSearch..321/5,9R,9A,DIG.1; 323/4355; 307/227, 221, 223; 328/186 Marzolf [451Mar. 28, 1972 [54] SWITCHING INVERTER WITH SINE 3,321,693 5/1967Heinrich et al. ..321/s E UT UT 3,514,688 5/1970 Martin ..321/9 x I3,227,889 1/1966 Paynter ....3o7/227 x [721 Mamlf Falls Chm, 3,329,8317/1'967 Abramson et al. ..307/223 B [73] Assignee: The United States ofAmerica as represented by the Secretary 0 the United I FOREIGN PATENTSOR APPLICATIONS slates y 680,926 2/1964 Canada ..32l/DlG. 1 22 FilediSept. 30, 1970 1,339,607 l/1966 France ..321/5 OTHER PUBLICATIONSElectronic Design pp. 74- 77, Jan. 20, 1964.

and Sol Sheinbein [57] ABSTRACT [56] References Cited A technique ofproducing a sine wave output from a solid state UNITED STATES PATENTSinverter using Triacs as switches to supply the sine wave output from aDC input. 3,470,449 9/1969 Risberg.; ..32l/l1 6 3,482,] 14 12/ i969Marshall ..307/22l B 2 Claims, 8 Drawing Figures /2 SEQUENCING C I R CUl T /l /3 0c POWER AC 3111 INPUT 'NVERTOR SWITCHES WAVE OUTPUT OUTPUTFROM BASIC INY PATENTED MAR 2 8 I972 SHEET 1 [IF 2 l2 seousncme cmcun /3F IG. 0c POWER AC sm INPUT 'NVERTOR SWITCHES WAVE OUTPUT TO SEOUENCINGCIRCUIT (FIG.3)

1 24T 35% OUTPUT OUTPUT No.l No.3 $25 L 1 COMMON 20 g 0 30 TO No.4 1 T0No.9

OUTPUT OUTPUT No.2 No.l0 "if 28 INVENTOR. FIG. 3 JOSEPH M MARZOLF BYfiAGENT ATTORNEY PATENTEUMARZB I912 3,652,918

' SHEUEUFZ TO YSEQUENCING c'mcuns OUTPUT wmomes FROM BASIC ,3 INVERTER(FIG.2)

(REVERSED) SWITCHING INVERTER WITH SINE WAVE OUTPUT STATEMENT OFGOVERNMENT INTEREST The invention described herein may be manufacturedand used by or for the Government of the United States of Amerroyaltiesthereon or therefor.

BACKGROUND OF THE INVENTION Semiconductor devicesare widely used in DCto AC inverters and DC to DC converters and are most conveniently andefficiently employed as switches. This method of employment inherentlyproduces square wave outputs. When used for DC to DC converters, thischaracteristic is desirable since it produces minimum ripple in therectified DC output. However many electrical loads are designed for sinewave inputs, which cannot be efficiently supplied by square loopmagnetic cores and semiconductor switches. I Many attempts have beenmade to overcome this incompatibility by using multiple switches toapproximate a sine wave output. Invariably, however, such attempts haveevolved such complex circuits as to be unacceptable from the standpointof reliability and cost. This complexity is caused by the need for areasonably large number of steps (and therefore switches) to produce anacceptable sine wave and, also, since most semiconductor devices, suchas SCRs are unidirectional,

different switches must be used for the positive and negative halfcycles of the sine wave. Such circuits can very quickly becomeincredibly complex. In any event, all such circuits represent a tradeoffbetween complexity and quality of the sine wave.

SUMMARY OF THE INVENTION This invention has all the advantages ofsimilar prior art devices while requiring a considerably smaller numberof component parts. It provides for a relatively simple method forobtaining a multiple stepped sine wave output using semiconductordevices as switches. An inverter is used to provide multiple voltage toTriacs which are then sequenced to supply the proper voltages to theload by a sequencing circuit which is controlled by the inverter.

OBJECTS OF THE INVENTION It is therefore an object of the presentinvention to provide a sine wave output from an inverter utilizing fewercomponents.

A further object of the present invention is to provide a sine waveoutput from a DC source that requires less filtering.

A still further object of the present invention is to provide aswitching inverter that is smaller and lighter than conventionalinverters.

. Yet another object of the present invention to provide a DC to ACinverter with an approximate sine wave output using multiple switches ina relatively simple manner.

These and other objects of the present invention will More specifically,the inverter is a simple conventional square wave static inverter thatis used universally for raising or lowering DC voltages and is sometimescalled the Royer circuit. Referring now to FIG. 2, the inverter includesa center tapped primary winding 14 and transistors 15 and 16 which areused as switches; The DC input voltage at terminal 17 is alternatelyswitched through each half of the winding 14 to reverse the coremagnetization each half cycle. Current flowing through the upper half ofthe primary winding 14 induces a voltage in the upper winding 18 todrive transistor 15 on and maintain current flow through the upper halfof the primary winding. At-the same time, a voltage is.induced in thelower winding 19 with the proper polarity to keep the lower transistor16 off. When the core 14 saturates, the induced voltages in bothwindings 18 and 19 go to zero and. the transients cause transistor 15 tocut off and transistor 16 to go on, thus reversing the induced voltagein all coils. This con- I tinues until the core 14 saturates in theopposite direction causing transistors 15 and 16 to switch back to theiroriginal states and complete one cycle of the inverter operation.

The voltage induced in secondary windings 20, 21, 22 and 23 arealternating square waves which can have any desired magnitude dependingon the number of turns on the secondary winding. For the example shownin FIG. 2 for illustrative purposes, a DC voltage of 7.5 volts atterminal 17 and with 18 turns in winding 18, 72 turns in winding 14, 18turns in winding 19, 48 turns in winding 20, 28 turns in winding 21, 64turns in winding 22, and 78 turns in winding 23, will produce outputvoltages of approximately 5.4 volts on winding 21, 12.4 volts on winding22, and 15.2 volts on winding 23.

The upper center tapped winding 20 is used to drive the sequencingcircuit of FIG. 3. The alternating square wave output of winding 20makes the upper and lower busses alternating positive and negative withrespect to the center tap bus at the frequency of the inverter. Morespecifically, FIG. 3 is drawn to illustrate a ten stage SCR circuit,though only stages 1,2,3 and 10 are shown in the Figure.

When the first stage fires by triggering on the silicon controlledrectifier 24, current flows from the center bus through resistor 25 andSCR 24. This current causes a negative voltage with respect to thecenter bus to appear at the Output No. 1 terminal and this voltage isused to trigger Triac 26 of FIG. 4. At the same time, current flowsthrough resistor 27 of the second stage of the sequencing circuit andSCR 24. The voltage appearing across resistor 27 causes capacitor 28, onthe order of 0.1 ,uf. to charge. This continues until the basic inverterreverses the polarity of its output. Since its output is reversed, SCR24 in the first stage stops conducting and SCR 29 of the second stagehas the correct polarity for operation. It

' does not conduct, however, until triggered on when its gate is becomemore apparent upon consideration of the specifica- DESCRIPTION OF THEPREFERRED EMBODIMENT Referring now to FIG. I, the block diagramcomprises three basic functional circuits, inverter 11, sequencingcircuit 12, and power switches 13. A DC input to the inverter 11provides multiple voltage outputs to power switches 13 and the power andtiming intervals to sequencing circuit 12 which in turn activates thepower switches to supply the proper AC sine wave output to the load inthe proper sequence.

supplied by the discharge of capacitor 28. The second stage operateswith current flowing down through resistor 30 and SCR 29. This producesa negative voltage at Output No. 2 with respect to the center bus whichis supplied as a triggering signal to Triac 31 of FIG. 4. Similarly,this voltage charges capacitor 32 of the third stage which will continueuntil the inverter reverses its polarity. This action is repeated foreach stage in succession, and the tenth stage is connected back on lead33 to fire the first stage so that the process continues repeatedly,firing each stage in succession. Capacitor 34, on the order of 1 p.f.,in series with diode 35, provides the triggering signal to the firststage when the circuit is first turned on. Capacitor 34 charges andstays charged as long as the sequencing circuit is in operation.

Referring now to FIG. 4, winding 21, 22 and 23, previously described inFIG. 2 supply the voltages through Triacs 26, 31 and 36. The lowerterminals of each goes to a common bus and the upper terminal goes to aTriac switch. The common bus of FIG. 4 is coupled to the common bus ofFIG. 3. Resistors 37 through 46 on the order of 200 ohms, are coupled tothe output terminals of the 10 stages of the sequencing circuit of FIG.3 in the order shown in order to provide the proper sequencing fortriggering the Triac 26, 31 and 36 to sequentially connect the properoutput from the inverter to the load 50. These Triacs operate asbidirectional switches and con duct with either polarity when triggeredon. Coils 47, 48 and 49, between the Triacs and windings serve toprovide some inductance in the circuit and to limit the very rapid rateof increase in voltage and current across the Triacs at the time ofswitching, thereby preventing the Triacs from firing spontaneouslywithout a triggering signal. Coils 47, 48 and 49 saturate quickly andthus do not add appreciable impedance to the circuitry after becomingsaturated.

The manner in which the sine wave is obtained can best be understood byreferring to FIG. 5. The outputs of the three power output circuits fromthe basic inverter are shown in a, 5b, and 56 showing their relativetiming and magnitude. These three outputs are shown in the powerswitching circuit (FIG. 4) and are each connected to the output busthrough a Triac, but only one Triac is activated at any given time. Notealso that the polarity of the winding 22 in FIG. 4 is reversed.Therefore, the output shown in FIG. 5b has been reversed from thoseshown in FIG. 5a and 5c. Thus by taking pulses successively from a, b,c, b, a, a, b, c, b, a, the synthesized curve shown in FIG. 5d isobtained. This selection is accomplished by selectively triggering theproper Triac to conduct during each half cycle supplying the propervoltage to the load 50.

An improved sine wave simulation can be made by using a larger number ofsteps without much increase in complexity. The number of steps N isgiven by the expression 4a+2, where a is any desired positive integer.The corresponding number of outputs required from the basic inverter(and also the number of Triacs) is given by the expression (N+2)/4,where N is the number of steps. The basic inverter frequency equals N/2times the frequency of the desired sine wave output. All evennumberedoutputs are connected in the reverse polarity in the power switchingcircuit.

The use of a larger number of steps may be quite practical. With theTriacs simplifying the power switching functions because they arebidirectional and also make simpler trigger circuits possible, the chiefcomplexity lies in the sequencing circuit. The circuit shown in FIG. 3was constructed of discrete components. It is really only a lO-stageshift register to supply triggering signals to the Triacs. With theadvent of integrated circuits in the logic field, such devices areprobably already available commercially as complete units. Therefore, itshould be a relatively simple matter to utilize them directly in lieu ofthe sequencing circuit explained above. If such devices are available,it is possible to increase the number of steps to produce a much higherquality sine wave without materially adding to the complexity.

The technique outlined above is a relatively simple method of producinga sine wave output from a solid-state inverter using semiconductordevices in the most efficient manner (as switches). The power switchingtechnique using Triacs requires only (N+2 )/4 switches, as compared to Nswitches in conventional circuits. If integrated circuits are employedfor the shift-register sequencing circuit, it is entirely practical toproduce a reliable many-step sine wave output of high quality, usingapproximately one-fourth the power switches normally required. Since thebasic inverter for a sine wave output with a large number of steps wouldbe operating at a relatively high frequency, the size and weight of theinverter would be very greatly reduced because smaller cores would berequired. Since the output wave closely approximates a sine wave inform, the amount of filtering required to produce an excellent sine waveoutput would be a minimum, thus further reducing size, weight, andfiltering losses over those of more conventional inverters. The use ofintegrated circuit switching reduces the size and weight considerablyover that of a conventional inverter of the same rating.

It is understood, of course, that the foregoing disclosure isspecifically directed to selected embodiments which are preferredto'cover all modifications and changes of the embodiments disclosedwhich do not depart from the spirit and scope of the invention.

I claim: 1. Apparatus for producing a sine wave output voltage from a DCinput voltage comprising:

an inverter;

a sequencing circuit coupled to said inverter;

a switching circuit comprising a plurality of bidirectional switchescoupled to said sequencing circuit and said inverter for producing saidsine wave output, said switches being triggered by said sequencingcircuit; and

an output load coupled to said switching circuit for receiving said sinewave output;

wherein the improvement comprises:

- A/C means for supplying an alternating current (A/C) voltage which iscoupled to said sequencing circuit;

said sequencing circuit comprising a plurality of stages coupledtogether in a series manner such that each stage is triggered on, only,when the previous stage has just previously been conductive and whensaid A/C voltage changes polarity;

each of said stages comprising an SCR having an anode connected to oneend of a load which produces a signal to trigger said bidirectionalswitches, said anode being connected to a first lead of an isolatedcapacitor which charges to a potential when said SCR is conductive;

a second lead of said capacitor being coupled to the gate of the nextstages SCR;

said potential on said capacitor disposing said next stage's SCR tobecome conductive upon the changing of the A/C voltage polarity;

said SCR and said next stages SCR being disposed to conduct on oppositepolarities of said A/C voltage.

2. The apparatus defined in claim 1 wherein said A/C means comprises acenter tap output winding ofsaid inverter;

said center tap having the other end of said load of each stage of saidsequencing circuit connected thereto;

said output winding having first and second leads which are opposite inA/C polarity;

said first lead being connected the cathode of to said SCR and saidsecond lead being connected to said next stages SCR.

1. Apparatus for producing a sine wave output voltage from a DC inputvoltage comprising: an inverter; a sequencing circuit coupled to saidinverter; a switching circuit comprising a plurality of bidirectionalswitches coupled to said sequencing circuit and said inverter forproducing said sine wave output, said switches being triggered by saidsequencing circuit; and an output load coupled to said switching circuitfor receiving said sine wave output; wherein the improvement comprises:A/C means for supplying an alternating current (A/C) voltage which iscoupled to said sequencing circuit; said sequencing circuit comprising aplurality of stages coupled together in a series manner such that eachstage is triggered on, only, when the previous stage has just previouslybeen conductive and when said A/C voltage changes polarity; each of saidstages comprising an SCR having an anode connected to one end of a loadwhich produces a signal to trigger said bidirectional switches, saidanode being connected to a first lead of an isolated capacitor whichcharges to a potential when said SCR is conductive; a second lead ofsaid capacitor being coupled to the gate of the next stage''s SCR; saidpotential on said capacitor desposing said next stage''s SCR to becomeconductive upon the changing of the A/C voltage polarity; said SCR andsaid next stage''s SCR being desposed to conduct on opposite polaritiesof said A/C voltage.
 2. The apparatus defined in claim 1 wherein saidA/C means comprises a center tap output winding of said inverter; saidcenter tap having the other end of said load of each stage of saidsequencing circuit connected thereto; said output winding having firstand second leads which are opposite in A/C polarity; said first leadbeing connected the cathode of to said SCR and said second lead beingconnected to said next stage''s SCR.